1. Field of the Invention
The present invention relates to a single axis multistage centrifugal compressor including diffusers provided radially outwardly from impellers of the compressor.
Multistage centrifugal compressors are of a high-pressure type and, typically, have an operating outlet pressure of at least 50 atmospheres, with such compressors being used for compressing gases, for example, the chemical industry for injecting gases, in oil fields for air compression and in gas pipe lines.
2. Description of the Prior Art
High-pressure high-speed multistage centrifugal compressors have been proposed wherein a plurality of centrifugal compressor stages are arranged at one rotational axis, with the centrifugal compressor utilizing vaneless diffusers; whereas, in low-pressure compressors vaned diffusers are widely used both in the single stage and multistage compressors. In these compressors, the ratio r/R of an outlet radius of impeller R to the diffuser vane leading edge radius r is constant at all stages. Multistage centrifugal compressors of the aforementioned type are described in, for example, "Blower and Compressor", Takefumi Ikui, Asakura Shoten, June 25, 1974.
Diffusers with vanes have not been adopted for high-pressure compressors, because a multistage centrifugal compressor having vaned diffusers has a high maximum efficiency but a narrow operating range. As fluid is compressed in the multistage centrifugal compressor, the passage width becomes smaller toward the delivery side of the compressor since its volume flow rate becomes smaller. As a result, the specific speed of an impeller at rear stages is smaller than that of an impeller at front stages. Thus, the pressure is higher and the specific speed is smaller at a rear stage in a multistage centrifugal compressor, and the phenomenon called "rotating stalls" often occurs at the rear stage side of compressor.
A rotating stall in a vaneless diffuser is generated when the flow is reduced in a certain compressor stage and the average flow angle .alpha. at a diffuser inlet of the stage becomes less than the prescribed value. Under these conditions, because of the rise of static pressure in the radial direction, reverse flow initiates locally at the boundary layers in the diffuser passage and develops into the main flow. The stall area rotates around the axis at low frequency.
If such a rotating stall occurs, pressure fluctuation caused by the stall becomes a strong shaft exciting force, as the pressure of the fluid increases. Accordingly, shaft vibration becomes large in a compressor with a high pressure level and driving the compressor becomes difficult, which limits the operating range of the compressor.
For example, if a rotating stall occurs at point C in a vaneless diffuser stage as shown in the graph of FIG. 8, the stable driving range (SDR) is at a larger flow rate than Qc. This means that the operating range becomes narrow compared with the case where a rotating stall is assumed not to occur, in which stable operation is possible at a flow rate greater than Qa.
This problem of vibrations in very high pressure centrifugal compressors was discussed by Ferrara in American Society of Mechanical Engineering (ASME) publication 77-DET-15 of 1977, and attributed to rotating stalls.
Turusaki in the Japanese magazine "Turbomachine" Vol. 12, 1984, No. 6, pages 323-332, describes rotating stalls in more detail, and Nishida et. al. in Reports of the Japanese Society of Mechanical Engineering, March 1988, pages 589-594 discuss the conditions for rotating stalls in vaneless diffusers.
As a countermeasure against rotating stalls in a vaneless diffuser, it has been usual to reduce the axial passage height of the diffuser from h to h' as shown in FIGS. 9 and 10 to delay the onset of a rotating stall. The ratio of the diffuser passage height to the outlet height b of the impeller is reduced and the radial velocity is increased from Cm to Cm'. The flow angle .alpha.' at the diffuser inlet is thus larger compared with the flow angle .alpha. when the diffuser passage height is larger. Accordingly, it is possible to widen the stable driving range as graphically shown in FIG. 11 by enlarging the inlet flow angle for the same flow rate to delay the onset of a rotating stall. Thus, reducing the passage height has the effect of moving the rotating stall onset point towards the surge point. Rotating stalls can be prevented if the diffuser passage height is decreased greatly compared with the impeller outlet height. However, it is necessary drastically to lower the diffuser passage height completely to prevent rotating stalls, and as the average fluid velocity becomes large in addition to the reduction in passage height by this method, friction loss in the diffuser is increased and performance becomes lower.